System and method for providing preventive overdrive pacing and antitachycardia pacing using an implantable cardiac stimulation device

ABSTRACT

Techniques for enabling both preventive overdrive pacing and antitachycardia pacing (ATP) within an implantable device are provided. The device gains the benefits of overdrive pacing for preventing the onset of a tachycardia and, if one nevertheless occurs, ATP is employed to terminate the tachycardia. In particular, a technique is provided for promptly detecting the onset of atrial tachycardia during preventive overdrive pacing based on loss of capture of atrial pacing pulses. A technique is also provided for using detection of loss of capture of atrial or ventricular pacing pulses to trigger automatic switching from overdrive pacing to ATP. A setup technique determines whether to enable the automatic switching from overdrive pacing to ATP within a particular patient. Also, techniques are provided for verifying loss of capture of atrial or ventricular backup pacing pulses and for detecting low amplitude ventricular fibrillation based on loss of capture of ventricular backup pacing pulses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to copending U.S. patent applications: 1)Ser. No. 10/657,858, titled “System and Method for Providing PreventiveOverdrive Pacing and Antitachycardia Pacing Using an Implantable CardiacStimulation Device”; 2) Ser. No. 10/657,897, titled “System and Methodfor Providing Preventive Overdrive Pacing and Antitachycardia PacingUsing an Implantable Cardiac Stimulation Device”; 3) Ser. No.10/657,963, titled “System and Method for Providing Preventive OverdrivePacing and Antitachycardia Pacing Using an Implantable CardiacStimulation Device”; and 4) Ser. No. 10/657,840, titled “System andMethod for Providing Preventive Overdrive Pacing and AntitachycardiaPacing Using an Implantable Cardiac Stimulation Device”; allapplications filed concurrently herewith.

FIELD OF THE INVENTION

The invention generally relates to implantable cardiac stimulationdevices such as pacemakers or implantable cardioverter defibrillators(ICDs), and, in particular, to techniques for overdrive pacing hearttissue to prevent tachycardia and for performing antitachycardia pacingto terminate tachycardia.

BACKGROUND OF THE INVENTION

An arrhythmia is an abnormal heart beat pattern. One example ofarrhythmia is bradycardia wherein the heart beats at an abnormally slowrate or wherein significant pauses occur between consecutive beats.Other examples of arrhythmias include tachyarrhythmias wherein the heartbeats at an abnormally fast rate. With atrial tachycardia, the atria ofthe heart beat abnormally fast. With ventricular tachycardia, theventricles of the heart beat abnormally fast. Though often unpleasantfor the patient, a tachycardia is typically not fatal. However, sometypes of tachycardia, particularly ventricular tachycardia, can triggerventricular fibrillation wherein the heart beats chaotically such thatthere is little or no net flow of blood from the heart to the brain andother organs. Ventricular fibrillation, if not terminated withinminutes, is fatal. Hence, it is highly desirable to prevent or terminatearrhythmias, particularly arrhythmias of the type that can lead to aventricular fibrillation.

One technique for preventing tachycardias is to pace the heart at a ratesomewhat faster than the intrinsic heart rate of the patient using atechnique referred to as overdrive pacing. To help prevent a tachycardiafrom occurring, the stimulation device artificially paces the heart atan overdrive rate set to be slightly faster than the intrinsic heartrate of the patient. One particularly effective overdrive pacingtechnique, referred to herein as dynamic atrial overdrive (DAO) pacing,is described in U.S. Pat. No. 6,519,493 to Florio et al., entitled“Methods And Apparatus For Overdrive Pacing Heart Tissue Using AnImplantable Cardiac Stimulation Device,” which is incorporated byreference herein. With DAO, the overdrive rate is controlled to remaingenerally uniform and, in the absence of a tachycardia, is adjustedupwardly or downwardly only occasionally. Dynamic overdrive techniquesare also applicable to the ventricles and exemplary dynamic ventricularoverdrive (DVO) techniques are described in U.S. patent applications: 1)Ser. No. 10/456,060 to Park et al., entitled “System And Method ForDynamic Ventricular Overdrive Pacing,” filed Jun. 6, 2003; and 2)10/456,058, entitled “System And Method For Dynamic VentricularOverdrive Pacing,” Jun. 6, 2003, which applications are alsoincorporated herein by reference.

It is believed that DAO and DVO are effective for at least some patientsfor preventing tachycardia for the following reasons. A normal, healthyheart typically beats only in response to electrical pulses generatedfrom a portion of the heart referred to as the sinus node. The sinusnode pulses are conducted to the various atria and ventricles of theheart via certain, normal conduction pathways. In some patients,however, additional portions of the heart also generate electricalpulses referred to as “ectopic” pulses. Each pulse, whether a sinus nodepulse or an ectopic pulse has a refractory period subsequent theretoduring which time the heart tissue is not responsive to any electricalpulses. A combination of sinus pulses and ectopic pulses can result in adispersion of the refractory periods, which, in turn, can trigger atachycardia. By overdrive pacing the heart at a generally uniform rateslightly above the intrinsic rate, the likelihood of the occurrence ofectopic pulses is reduced and the refractory periods within the hearttissue are rendered more uniform and periodic. Thus, the dispersion ofrefractory periods is reduced and the risk of tachycardia is reduced.Thus, overdrive pacing, particularly DAO and DVO, provides a usefultechnique for helping to prevent the onset of a tachycardia and forterminating a tachycardia should one nevertheless arise. Utilizing anoverdrive algorithm in conjunction with multisite stimulation will alsoresult in a collision of the electrical wavefronts further reducingdispersion of the refractory period effectively reducing the risk oftachycardia.

Herein, the term “overdrive pacing” generally refers to the sustainedpacing of chambers of the heart at a rate higher than the intrinsicrate. Overdrive pacing can take the form of “preventive overdrivepacing”, which is employed for the purposes of preventing a tachycardiafrom occurring, and “therapeutic overdrive pacing”, which is employedfor the purposes of terminating a tachycardia should one neverthelessarise. The overdrive rates associated with preventive overdrive pacingare much lower than those associated with therapeutic overdrivingpacing.

Therapeutic overdrive pacing represents one type of antitachycardiapacing (ATP). Other ATP techniques have been developed as well that donot exploit overdrive pacing or at least do not exploit sustainedoverdrive pacing. The underlying principle of many such techniques isthat if an implantable stimulation device delivers a stimulation pulseto the heart during a critical time period following a naturallyoccurring heartbeat during tachycardia, the tachycardia pathway will berendered refractory abruptly terminating the tachycardia allowing theheart to revert to sinus, or natural, rhythm. In this regard, certaintypes of tachycardias are the result of an electrical feedback mechanismwithin the heart. For example, a natural heartbeat can occur through anormal pathway and re-enter through an alternate loop of tissue thatperpetuates conduction (also known as an accessory or re-entrantpathway), thereby initiating a tachycardia. The delivery of astimulation pulse causes the cardiac tissue in front of the stimulationpulse to depolarize (thereby causing the heart to contract), but leavesthe tissue at the stimulation site refractory (i.e., the tissue cannotrespond to additional stimulation). Thus, by injecting a stimulationpulse within the cardiac cycle, the stability of the feedback loop isdisrupted and the tachycardia terminated thus allowing the heart mayrevert to a natural sinus rhythm.

One example is burst pacing (or “shotgunning”) wherein severalsequential, rapid stimuli are delivered to the heart in an effort toterminate the tachycardia. The theory behind providing a burst of pulsesis that sooner or later one of the stimulating pulses will occur at atime in the tachycardia cycle which will terminate the tachycardia (i.e.the pulse will occur during a “region of susceptibility”). Burst pacingmay be delivered asynchronously or synchronously at a fixed, decreasing,or increasing, cycle length from a tachycardia complex until thetachycardia is terminated. Once the tachycardia has been terminated, thetiming associated with the burst that succeeded in terminating thetachycardia may be stored and used as the starting point for applying anew burst of simulation pulses to the heart upon the next occurrence ofa tachycardia. An alternate technique to find the termination window isby “scanning”, which is a type of burst pacing with variations incoupling interval. This technique utilizes an implantable stimulationdevice that automatically searches or “scans” for the pacing intervalmost likely to terminate a tachycardia. The implantable stimulationdevice delivers single or multiple stimulation pulses at “criticallytimed” coupling intervals and continues in a controlled sequence untilthe tachycardia terminates. For example, the controlled sequence maybegin with a single stimulation pulse at one end of the scanning windowand, with each successive tachycardia cycle, deliver additional pulsesat increasing (or decreasing) coupling intervals in a controlled mannertowards the other end of the window. Hence, the stimulation pulse scansthrough the scanning window looking for the region of susceptibility.

Exemplary patents describing ATP techniques include U.S. Pat. No.6,101,414, to Mark Kroll, entitled “Method And Apparatus ForAntitachycardia Pacing With An Optimal Coupling Interval,” and U.S. Pat.No. 5,431,689 to Weinberg et al., entitled “Implantable StimulationSystem And Method For Terminating Cardiac Arrhythmias,” which are bothincorporated by reference herein.

Regardless of the specific ATP technique, it has been found that ATP ismost effective if applied early during the tachycardia. Unfortunately,conventional techniques for detecting the onset of a tachycardia do notdetect the tachycardia as promptly as would be desired. One techniquefor detecting an atrial tachycardia is to monitor the atrial rate andinitiate atrial ATP if the heart rate exceeds a certain threshold,typically referred to as an atrial tachycardia detection rate (ATDR). Itmay take a fair number of cardiac cycles, however, before thestimulation device can reliably detect a high atrial rate and, inparticular, distinguish a high heart rate from a temporary shortening ofan atrial heart rate interval caused by a premature beat such as apremature atrial contraction (PAC). It is also known to try todifferentiate pathologic rhythms from normal physiologic rhythms byanalyzing heart rate stability. Again, though, a fair number of cyclesmay be required before the stimulation device can reliably distinguish achange in heart rate stability caused by a tachycardia from one causedby premature beats or other transient factors. Conventional techniquesfor detecting a tachycardia are discussed in U.S. Pat. No. 5,109,842, toAdinolfi, entitled “Implantable Tachyarrhythmia Control System Having aPatch Electrode with an Integrated Cardiac Activity System.”

Also, care must be taken to ensure that ATP is not erroneously activatedin circumstances where it is not needed and, in particular, incircumstances where it might be proarrhythmic. In this regard, in somepatients, there are a large number of short nonsustained salvos ofsupraventricular tachycardia (SVT) or multiple sequential PACs which, ifthey occur during a post-ventricular atrial refractory period (PVARP),will not inhibit the atrial output thereby causing the atrial output tobe delivered to a period of physiologic refractoriness in the atrialmyocardium. Indeed, delivering a burst of ATP into a rhythm that wouldnot have been sustained may be arrhythmogenic inducing atrial flutter oratrial fibrillation (AF) where this would not have occurredspontaneously. The need to avoid erroneous triggering of ATP often meansthat the tachycardia detection technique must process even more databefore reliably concluding that a tachycardia has occurred.

Similar problems can arise in the detection of certain ventriculartachycardias. Failure to promptly detect a ventricular tachycardia (suchas a low amplitude ventricular fibrillation (VF)) can result in a delayin the delivery of defibrillation shocks with a reduced likelihood ofsuccess. In this regard, it has been proposed to use the detection ofloss of capture (LOC) of a series of ventricular pacing pulses as ameans for detecting low amplitude VF and for triggering delivery of ahigh output defibrillation shock. See U.S. Pat. No. 5,350,401 to Levine,which is incorporated herein by reference. With that technique, upondetection of loss of capture of a ventricular pulse, the ventricularpulse output magnitude is increased and another pulse is delivered. Ifthat pulse also fails to capture, the output magnitude is increasedagain. This process proceeds until either a ventricular pulse capturesor until a maximum pulse output level is reached. If the maximum outputis reached and the ventricular pulses still do not evoke capture, adetermination is thereby made that a low amplitude VF may have occurredand a defibrillation shock may be delivered to terminate the VF.Although the technique is effective in eventually detecting lowamplitude VF, the need to deliver a series of ventricular pulses withdifferent pulse magnitudes delays the detection of VF, thus potentiallyreducing the effectiveness of subsequent shock therapy.

Thus, conventional atrial and ventricular tachycardia detectiontechniques do not always detect tachycardia as quickly as desired,resulting in a reduced likelihood that subsequent therapy will besuccessful. Accordingly, it would be desirable to provide improvedtechniques for promptly and reliably detecting atrial and ventriculartachycardias and aspects of the invention are generally directed tothese ends. In particular, it would be desirable to provide techniquesfor exploiting the detection of loss of capture of backup pacing pulsesin the detection of tachycardia. As will be explained below, loss ofcapture of backup pulses can be used to promptly detect a tachycardia.

The aforementioned problems associated with promptly detecting atachycardia are even more problematic if preventive overdrive pacingwere also to be performed. For example, in the technique where ATP istriggered based on intrinsic atrial rate, routine preventive overdrivepacing would prevent the device from continuously and reliablymonitoring the intrinsic atrial rate. Using DAO, for example, theintrinsic atrial rate is not evaluated at all. Rather, increases ordecreases in the overdrive rate are made based solely on the presence orabsence of breakthrough beats. Alternatively, ATP could be triggeredonce the preventive overdrive pacing rate itself exceeds the ATDR, butit may take too many cycles before the overdrive rate is boosted up tothat threshold. Also, as typically implemented, preventive atrialoverdrive pacing rates never exceed the ATDR because the pacing deviceis programmed to never exceed a maximum overdrive rate that is set wellbelow the ATDR. Likewise, techniques based on heart rate stability arenot easily and effectively implemented during preventive overdrivepacing.

For these and other reasons, most conventional pacing devices do notprovide for both preventive overdrive pacing and ATP. In devices that doprovide for both (see, for example, the AT500 pacemaker provided byMedtronic Corporation of Minneapolis, Minn., USA), it does not appearthat ATP is triggered as promptly as desired and so may not provideoptimal termination of the tachycardia. Accordingly, it also would bedesirable to provide improved techniques promptly switching frompreventive overdrive pacing to ATP and additional aspects of theinvention are directed to these ends.

In view of the risks associated with delivering ATP during arrhythmiasthat otherwise would not be sustained (such as where burst ATP mightactually induce atrial flutter or fibrillation), it also would bedesirable to provide techniques for determining whether to enableautomatic switching of preventive overdrive pacing to ATP within aparticular patient and still other aspects of the invention are directedto that end.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the invention, systems and methodsare provided for use with implantable cardiac stimulation devices forpromptly and reliably detecting the onset of an atrial tachycardia. Inone embodiment, wherein the stimulation device includes a pacing unitand an atrial pulse capture detection unit, pacing pulses are deliveredto the atria using the pacing unit and atrial tachycardia is detectedbased on the loss of capture of the atrial pacing pulses. By detectingatrial tachycardia based on loss of capture of atrial pacing pulses,rather than other, slower techniques, atrial tachycardia can be promptlydetected, thereby permitting ATP to be delivered as early as possiblewithin the episode of atrial tachycardia to provide the best possiblechance of success.

In one specific example, wherein atrial pacing pulses are delivered at amaximum pulse magnitude, atrial tachycardia is detected by identifyingloss of capture of a single one of the atrial pacing pulses or byinstead detecting loss of capture of both a primary pulse and a backuppulse, both delivered at the maximum magnitude. If primary pacing pulsesare instead delivered at a pulse magnitude less than a maximum pulsemagnitude, atrial tachycardia is detected by identifying loss of captureof both a primary pacing pulse and a subsequent backup pacing pulsedelivered at the maximum pulse magnitude. Alternatively, rather thanpredicating the detection of atrial tachycardia on a LOC of a singleprimary pacing pulse or on a LOC of a pacing pulse/backup pulse pair,atrial tachycardia can instead be detected by identifying some number ofLOCs out of a predetermined number of pulses or pulse pairs (i.e. bydetecting “x” LOCs out of the last “y” pulses). This helps prevent anycoincidental LOC due to a short salvo of rapid ventricular rates fromresulting in the erroneous detection of an atrial tachycardia. Also, inimplementations wherein pacing pulses are delivered at a pulse magnitudeless than a maximum pulse magnitude, the stimulation device preferablyincludes an automatic stimulation threshold search unit operative todetermine a capture threshold for setting the magnitude of the primary(i.e. non-backup) atrial pacing pulses. The magnitudes of the primarypulses are set to a working margin just above the determined capturethreshold. The threshold search is automatically performed whenever theprimary pacing pulse is not captured on two consecutive complexes butwhere the back-up pulses are captured. Threshold searches may beperformed periodically as well.

In accordance with a second aspect of the invention, systems and methodsare provided for use with implantable cardiac stimulation devices forproviding both preventive overdrive pacing therapy and ATP therapy. Inone embodiment, the stimulation device includes a preventive overdrivepacing unit operative to deliver overdrive pacing pulses to the heart,an ATP therapy unit operative to deliver antitachycardia pacing therapyto the heart and a tachycardia detection unit operative to detecttachycardia. A control unit is provided that is operative to switch frompreventive overdrive pacing to ATP upon detection of a tachycardia. Byproviding for both preventive overdrive pacing and ATP within astimulation device, the likelihood of onset of a tachycardia can bereduced by overdrive pacing but, if one should nevertheless occur, ATPcan be use to terminate the tachycardia. Preferably, tachycardias aredetected based on the loss of capture of pacing pulses in accordancewith the first aspect of the invention, such that ATP can be promptlyactivated to have the best likelihood of success. In one specificexample, the stimulation device also includes a PAC detection unit fordetecting PACs during preventive overdrive pacing and the control unitis also operative to switch from preventive overdrive pacing and to ATPupon the detection of a loss of capture of a backup pulse deliveredsubsequent to detection of a PAC during preventive overdrive pacing.

In accordance with a third aspect of the invention, systems and methodsare provided for use with implantable cardiac stimulation devices fordetermining whether to enable automatic switching from preventiveoverdrive pacing to ATP within a particular patient. In one embodiment,wherein the stimulation device includes an overdrive pacing unit, atachycardia detection unit, an ATP therapy unit, and a capture detectionunit, a setup technique is performed wherein preventive overdrive pacingis delivered to the heart and any loss of capture of pacing pulses isdetected. Any tachycardias occurring subsequent to a loss of capture aredetected using the tachycardia detection unit and a determination ismade, for each such tachycardia, whether the tachycardia spontaneouslyterminates. Then, automatic switching from preventive overdrive pacingto ATP therapy is selectively enabled based on a percentage ofspontaneously terminating episodes of tachycardia occurring subsequentto loss capture during preventive overdrive pacing.

Preferably, the percentage of spontaneously terminating episodes oftachycardia is compared against a predetermined threshold (such as 60%)and automatic switching from preventive overdrive pacing to ATP therapyis enabled only if the percentage does not exceed the predeterminedthreshold and is disabled otherwise. In this manner, loss of capture ofoverdrive pulses is only used to trigger an automatic switch frompreventive overdrive pacing to ATP within those patients in which mosttachycardias arising subsequent to a loss of capture do notspontaneously terminate. In patients in which such tachycardias doindeed spontaneously terminate, switching from preventive overdrivepacing to ATP is not enabled, at least based on loss of capture. Thisminimizes the delivery of ATP to patients for whom it is not necessaryand could be arrhythmogenic. In one specific example, the setuptechnique is performed for a predetermined period of time, for exampleone to two months, following implant of the stimulation device withinthe patient to permit a reliable determination to be made as thepercentage of tachycardia episodes that spontaneously terminatingfollowing loss of capture. Alternatively, the setup technique isperformed until a predetermined number of tachycardia episodes aredetected. In either case, the setup technique is preferably repeatedperiodically to account for any changes in the percentage of tachycardiaepisodes that spontaneously terminate within the patient.

In accordance with a fourth aspect of the invention, captureverification is applied to backup pacing pulses, whether in the atria orventricles. An implantable cardiac stimulation device is provided, whichincludes a pacing unit for delivering primary pacing pulses to theheart, a pulse capture detection unit operative to detect loss ofcapture of primary pacing pulses, and a backup pulse unit for deliveringbackup pulses to the heart upon detection of a loss of capture of aprimary pacing pulse. The capture detection unit is further operative todetect loss of capture of the backup pacing pulses. As noted above,detection of loss of capture of backup pulses in the atria may be usedas a basis for detecting an atrial tachycardia and for triggering ATP.Detection of loss of capture of backup pulses may be used in othercircumstances as well, such as for the detection of low amplitude VFbased on loss of capture of ventricular backup pulses.

In accordance with a fifth aspect of the invention, loss of capture ofbackup pacing pulses in the ventricles is used to detect ventriculartachycardia, particularly low amplitude VF. Briefly, an implantablecardiac stimulation device is provided, which includes a pacing unitoperative to deliver primary pacing pulses and backup pacing pulses tothe ventricles of the heart and a capture detection unit operative todetect loss of capture of both primary pacing pulses and backup pacingpulses in the ventricles. A capture-based ventricular tachycardiadetection unit is also provided, which is operative to detect aventricular tachycardia based upon loss of capture of a ventricularbackup pulse as detected by the capture detection unit.

Thus, a variety of techniques are provided for promptly detecting atrialand ventricular tachycardias and for enabling use of both preventiveoverdrive pacing and ATP within implantable stimulation devices. Otherfeatures, advantages and objectives of the invention will be apparentfrom the accompanying drawings in connection with descriptions providedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention may be more readilyunderstood by reference to the following description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a simplified diagram illustrating an implantable stimulationdevice in electrical communication with at least three leads implantedinto the heart of a patient for delivering multi-chamber stimulation andshock therapy and configured in accordance with the invention to performpreventive overdrive pacing and ATP;

FIG. 2 is a functional block diagram of the implantable cardiacstimulation device of FIG. 1 illustrating basic elements of astimulation device;

FIG. 3 is a flow chart providing an overview of the operation of anexemplary embodiment of the invention particularly illustrating themanner by which the implantable stimulation device of FIGS. 1 and 2detects an atrial tachycardia based upon loss of capture of pacingpulses;

FIG. 4 is a flow chart providing an overview of the operation of anotherexemplary embodiment of the invention and particularly illustrating themanner by which the implantable stimulation device of FIGS. 1 and 2automatically switches from preventive atrial overdrive pacing to ATP atthe onset of an atrial tachycardia;

FIG. 5 is a flow chart providing an overview of the operation of yetanother exemplary embodiment of the invention and particularlyillustrating the manner by which the implantable stimulation device ofFIGS. 1 and 2 determines, for a particular patient, whether to enablethe automatic switching technique of FIG. 4;

FIG. 6 is a flow chart providing an overview of the manner by which theimplantable stimulation device of FIGS. 1 and 2 verifies capture ofbackup pacing pulses; and

FIG. 7 is a flow chart providing an overview of still yet another of anexemplary embodiment of the invention and particularly illustrating themanner by which the implantable stimulation device of FIGS. 1 and 2detects ventricular tachycardia based upon loss of capture ofventricular backup pulse.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description includes the best mode presently contemplatedfor practicing the invention. The description is not to be taken in alimiting sense but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe ascertained with reference to the issued claims. In the descriptionof the invention that follows, like numerals or reference designatorswill be used to refer to like parts or elements throughout.

Overview of Implantable Device

As shown in FIG. 1, there is a stimulation device 10 in electricalcommunication with a patient's heart 12 by way of three leads, 20, 24and 30, suitable for delivering multi-chamber stimulation and shocktherapy. To sense atrial cardiac signals and to provide right atrialchamber stimulation therapy, the stimulation device 10 is coupled to animplantable right atrial lead 20 having at least an atrial tip electrode22, which typically is implanted in the patient's right atrialappendage, and an atrial ring electrode 23. To sense left atrial andventricular cardiac signals and to provide left chamber pacing therapy,the stimulation device 10 is coupled to a “coronary sinus” lead 24designed for placement in the “coronary sinus region” via the coronarysinus or for positioning a distal electrode adjacent to the leftventricle and/or additional electrode(s) adjacent to the left atrium. Asused herein, the phrase “coronary sinus region” refers to thevasculature of the left ventricle, including any portion of the coronarysinus, great cardiac vein, left marginal vein, left posteriorventricular vein, middle cardiac vein, and/or small cardiac vein or anyother cardiac vein accessible by the coronary sinus. Accordingly, anexemplary coronary sinus lead 24 is designed to receive atrial andventricular cardiac signals and to deliver left ventricular pacingtherapy using at least a left ventricular tip electrode 26, left atrialpacing therapy using at least a left atrial ring electrode 27, andshocking therapy using at least a left atrial coil electrode 28.

The stimulation device 10 is also shown in electrical communication withthe patient's heart 12 by way of an implantable right ventricular lead30 having, in this embodiment, a right ventricular tip electrode 32, aright ventricular ring electrode 34, a right ventricular (RV) coilelectrode 36, and an SVC coil electrode 38. Typically, the rightventricular lead 30 is transvenously inserted into the heart 12 so as toplace the right ventricular tip electrode 32 in the right ventricularapex so that the RV coil electrode will be positioned in the rightventricle and the SVC coil electrode 38 will be positioned in thesuperior vena cava. Accordingly, the right ventricular lead 30 iscapable of receiving cardiac signals, and delivering stimulation in theform of pacing and shock therapy to the right ventricle.

As illustrated in FIG. 2, a simplified block diagram is shown of themulti-chamber implantable stimulation device 10, which is capable oftreating both fast and slow arrhythmias with stimulation therapy,including cardioversion, defibrillation, and pacing stimulation. While aparticular multi-chamber device is shown, this is for illustrationpurposes only, and one of skill in the art could readily duplicate,eliminate or disable the appropriate circuitry in any desiredcombination to provide a device capable of treating the appropriatechamber(s) with cardioversion, defibrillation and pacing stimulation.

The housing 40 for the stimulation device 10, shown schematically inFIG. 2, is often referred to as the “can”, “case” or “case electrode”and may be programmably selected to act as the return electrode for all“unipolar” modes. The housing 40 may further be used as a returnelectrode alone or in combination with one or more of the coilelectrodes, 28, 36 and 38, for shocking purposes. The housing 40 furtherincludes a connector (not shown) having a plurality of terminals, 42,43, 44, 46, 48, 52, 54, 56, and 58 (shown schematically and, forconvenience, the names of the electrodes to which they are connected areshown next to the terminals). As such, to achieve right atrial sensingand pacing, the connector includes at least a right atrial tip terminal(A_(R) TIP) 42 adapted for connection to the atrial tip electrode 22 anda right atrial ring (A_(R) RING) electrode 43 adapted for connection toright atrial ring electrode 23. To achieve left chamber sensing, pacingand shocking, the connector includes at least a left ventricular tipterminal (V_(L) TIP) 44, a left atrial ring terminal (A_(L) RING) 46,and a left atrial shocking terminal (A_(L) COIL) 48, which are adaptedfor connection to the left ventricular ring electrode 26, the leftatrial tip electrode 27, and the left atrial coil electrode 28,respectively. To support right chamber sensing, pacing and shocking, theconnector further includes a right ventricular tip terminal (V_(R) TIP)52, a right ventricular ring terminal (V_(R) RING) 54, a rightventricular shocking terminal (R_(V) COIL) 56, and an SVC shockingterminal (SVC COIL) 58, which are adapted for connection to the rightventricular tip electrode 32, right ventricular ring electrode 34, theRV coil electrode 36, and the SVC coil electrode 38, respectively.

At the core of the stimulation device 10 is a programmablemicrocontroller 60, which controls the various modes of stimulationtherapy. As is well known in the art, the microcontroller 60 (alsoreferred to herein as a control unit) typically includes amicroprocessor, or equivalent control circuitry, designed specificallyfor controlling the delivery of stimulation therapy and may furtherinclude RAM or ROM memory, logic and timing circuitry, state machinecircuitry, and I/O circuitry. Typically, the microcontroller 60 includesthe ability to process or monitor input signals (data) as controlled bya program code stored in a designated block of memory. The details ofthe design and operation of the microcontroller 60 are not critical tothe invention. Rather, any suitable microcontroller 60 may be used thatcarries out the functions described herein. The use ofmicroprocessor-based control circuits for performing timing and dataanalysis functions are well known in the art.

As shown in FIG. 2, an atrial pulse generator 70 and a ventricular pulsegenerator 72 generate pacing stimulation pulses for delivery by theright atrial lead 20, the right ventricular lead 30, and/or the coronarysinus lead 24 via an electrode configuration switch 74. It is understoodthat in order to provide stimulation therapy in each of the fourchambers of the heart, the atrial and ventricular pulse generators, 70and 72, may include dedicated, independent pulse generators, multiplexedpulse generators, or shared pulse generators. The pulse generators, 70and 72, are controlled by the microcontroller 60 via appropriate controlsignals, 76 and 78, respectively, to trigger or inhibit the stimulationpulses.

The microcontroller 60 further includes timing control circuitry 79which is used to control the timing of such stimulation pulses (e.g.,pacing rate, atrio-ventricular (AV) delay, atrial interconduction (A—A)delay, or ventricular interconduction (V—V) delay, etc.) as well as tokeep track of the timing of refractory periods, blanking intervals,noise detection windows, evoked response windows, alert intervals,marker channel timing, etc., which is well known in the art. Switch 74includes a plurality of switches for connecting the desired electrodesto the appropriate I/O circuits, thereby providing complete electrodeprogrammability. Accordingly, the switch 74, in response to a controlsignal 80 from the microcontroller 60, determines the polarity of thestimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) byselectively closing the appropriate combination of switches (not shown)as is known in the art.

Atrial sensing circuits 82 and ventricular sensing circuits 84 may alsobe selectively coupled to the right atrial lead 20, coronary sinus lead24, and the right ventricular lead 30, through the switch 74 fordetecting the presence of cardiac activity in each of the four chambersof the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR.SENSE) sensing circuits, 82 and 84, may include dedicated senseamplifiers, multiplexed amplifiers, or shared amplifiers. The switch 74determines the “sensing polarity” of the cardiac signal by selectivelyclosing the appropriate switches, as is also known in the art. In thisway, the clinician may program the sensing polarity independent of thestimulation polarity. Each sensing circuit, 82 and 84, preferablyemploys one or more low power, precision amplifiers with programmablegain and/or automatic gain control, bandpass filtering, and a thresholddetection circuit, as known in the art, to selectively sense the cardiacsignal of interest. The automatic gain control enables the device 10 todeal effectively with the difficult problem of sensing the low amplitudesignal characteristics of atrial or ventricular fibrillation. Theoutputs of the atrial and ventricular sensing circuits, 82 and 84, areconnected to the microcontroller 60 which, in turn, are able to triggeror inhibit the atrial and ventricular pulse generators, 70 and 72,respectively, in a demand fashion in response to the absence or presenceof cardiac activity in the appropriate chambers of the heart.

For arrhythmia detection, the device 10 utilizes the atrial andventricular sensing circuits, 82 and 84, to sense cardiac signals todetermine whether a rhythm is physiologic or pathologic. As used herein“sensing” is reserved for the noting of an electrical signal, and“detection” is the processing of these sensed signals and noting thepresence of an arrhythmia. The timing intervals between sensed events(e.g., P-waves, R-waves, and depolarization signals associated withfibrillation which are sometimes referred to as “F-waves” or“Fib-waves”) are then classified by the microcontroller 60 by comparingthem to a predefined rate zone limit (i.e., bradycardia, normal, lowrate VT, high rate VT, and fibrillation rate zones) and various othercharacteristics (e.g., sudden onset, stability, physiologic sensors, andmorphology, etc.) in order to determine the type of remedial therapythat is needed (e.g., bradycardia pacing, antitachycardia pacing,cardioversion shocks or defibrillation shocks, collectively referred toas “tiered therapy”). Similar capabilities would exist on the atrialchannel with respect to tachycardias occurring in the atrium. Thesewould be atrial tachycardias (AT), more rapid atrial tachycardias(Atrial Flutter) and atrial fibrillation (AF).

Cardiac signals are also applied to the inputs of an analog-to-digital(A/D) data acquisition system 90. The data acquisition system 90 isconfigured to acquire intracardiac electrogram signals, convert the rawanalog data into a digital signal, and store the digital signals forlater processing and/or telemetric transmission to an external device102. The data acquisition system 90 is coupled to the right atrial lead20, the coronary sinus lead 24, and the right ventricular lead 30through the switch 74 to sample cardiac signals across any pair ofdesired electrodes.

The microcontroller includes a capture-based tachycardia detection unit99, which operates to detect a tachycardia based on loss of capture ofpacing pulses. In the primary example described herein, the tachycardiadetection unit operates to detect AF based on loss of capture of atrialpacing signals during preventive overdrive pacing in the atrium.Accordingly, the capture-based tachycardia detection unit is used inconjunction with a preventive overdrive pacing unit 101 for controllingoverdrive pacing of the heart. The overdrive pacing unit preferablyperforms preventive overdrive pacing in accordance with the DAOtechnique described above in the Background of the Invention section andset forth in the above-cited patent Florio et al. In one example, theoverdrive pacing unit operates continuously in the absence of atachycardia so as to reduce the likelihood of the onset of atachycardia. In other examples, preventive overdrive pacing is suspendedwhile the patient is asleep. It is also suspended when the patient is ina tachycardia that has resulted in the enabling of the Automatic ModeSwitch algorithm. In any case, if a tachycardia is detected duringpreventive overdrive pacing by the tachycardia detection unit, an ATPunit 103 is activated to deliver antitachycardia pacing to the heart inan effort to terminate the tachycardia. The ATP unit may administer ATPin accordance with any of a variety of ATP techniques, such as thetechniques described in the Kroll and Weinberg patents cited above. Aswill be descried in greater detail below in connection with FIG. 3 andfollowing, tachycardia may be detected during preventive overdrivepacing based, for example, upon detection of a true loss of capture ofan preventive overdrive pacing pulse or upon detection of a loss ofcapture of a backup pulse subsequent to a PAC.

To detect loss of capture, the microcontroller also includes anautomatic capture detection unit 105 for detecting an evoked responsefrom the heart in response to an applied stimulus. As will be explainedbelow, the capture detection unit verifies capture of both primarypacing pulses and any subsequent backup pulses. Capture occurs when anelectrical stimulus applied to the heart is of sufficient energy todepolarize the cardiac tissue, thereby causing the heart muscle tocontract. The capture detection unit detects a depolarization signalduring a window following a stimulation pulse, the presence of whichindicates that capture has occurred. Capture detection is performed on abeat-by-beat basis. If a primary pulse is not captured, a backup pulseunit 113 delivers a backup pulse at a maximum pulse magnitude. Thecapture detection unit also detects whether the backup pulse captures.

Also included is a stimulation threshold search unit 107 forautomatically determining the current capture threshold of the patient,i.e. the minimum output sufficient to evoke capture, so that the outputor pulse magnitude can be reset properly. This is commonly reported interms of pulse amplitude as this is one of the programmable outputparameters. As will be explained below, while preventive overdrivepacing is performed, is stimulation search is automatically performed incircumstances wherein a primary pacing pulse is not captured but thebackup pulse is captured. (If both the overdrive pulse and the backuppulse are not captured, ATP is instead activated.) Also, preferably, acapture threshold search is performed periodically to update the capturethreshold regardless of whether any loss of capture is detected. Suchcapture threshold searches are preferably performed every eight hours.Typically, a capture threshold search begins at a desired starting point(either a high energy level or the level at which capture is currentlyoccurring) and decreases the energy level until capture is lost. It thenincrements the output in 0.125 Volt steps until capture is restored. Thevalue at which capture is restored is known as the capture threshold.Thereafter, a working margin or a safety margin is added to the capturethreshold to yield a new pulse magnitude. A safety margin is a fixedmultiple of the measured threshold. A working margin is a fixed value,e.g. 0.25 Volts above the measured threshold. In the preferredimplementation, the safety margin is provided by the high output backuppulse. The delivered output associated with the primary pulse is simplya working margin above the measured capture threshold.

Various techniques for implementing capture verification of atrialpacing pulses (i.e. atrial AutoCapture) are set forth in U.S. Pat. No.6,434,428 to Sloman et al.; U.S. Pat. No. 6,311,089 to Mann et al.; U.S.Pat. No. 6,285,908 to Mann et al.; U.S. Pat. No. 6,263,244 to Mann etal.; U.S. Pat. No. 6,259,950 to Mann et al.; U.S. Pat. No. 6,243,606 toMann et al.; and U.S. Pat. No. 6,101,416 to Sloman, which areincorporated herein by reference. Capture verification of ventricularpulses is described in U.S. Pat. No. 6,456,882 to Schloss; U.S. Pat. No.6,456,881 to Bornzin et al.; and U.S. Pat. No. 6,345,201 to Sloman, etal, which are also incorporated herein by reference. See also U.S. Pat.No. 4,686,988 (Sholder); U.S. Pat. No. 4,969,467 (Callaghan et al.); andU.S. Pat. No. 5,350,410 (Mann et al.), which patents are herebyincorporated herein by reference. A technique for implementing automaticcapture verification during overdrive pacing is described in U.S. patentapplication Ser. No. 10/138,438, filed May 2, 2002, of Bradley et al.,entitled “Method And Apparatus For Providing Atrial Autocapture In ADynamic Atrial Overdrive Pacing System For Use In An Implantable CardiacStimulation Device,” which is incorporated herein by reference.

The microcontroller also includes a PAC detection unit 109 and a PACresponse unit 111. The PAC detection unit detects PACs (in accordancewith a technique described below in connection with FIG. 3 andfollowing) and the PAC response unit provides a pacing protocol forresponding to the PAC. An exemplary PAC response protocol is describedin U.S. Pat. No. 5,978,709 to Begemann et al., entitled “PacemakerSystem with Improved Techniques for Preventing and Suppressing AtrialArrhythmias,” which is incorporated herein by reference.

Although shown as being components of the microcontroller, any or all ofcapture-based tachycardia detection unit 99, overdrive pacing unit 101,ATP unit 103, capture detection unit 105, stimulation threshold searchunit 107, PAC detection unit 109, PAC response unit 111, and backuppulse unit could be instead implemented as separate components. Also,depending up on the particular component and the particularimplementation, individual components may be configured to apply to theventricles, the atria, or in some cases both.

The microcontroller 60 is further coupled to a memory 94 by a suitabledata/address bus 96, wherein the programmable operating parameters usedby the microcontroller 60 are stored and modified, as required, in orderto customize the operation of the stimulation device 10 to suit theneeds of a particular patient. Such operating parameters define, forexample, pacing pulse amplitude, pulse duration, electrode polarity,rate, sensitivity, automatic features, arrhythmia detection criteria,and the amplitude, waveshape and vector of each shocking pulse to bedelivered to the patient's heart 12 within each respective tier oftherapy. Other pacing parameters include base rate, rest rate andcircadian base rate.

Advantageously, the operating parameters of the implantable device 10may be non-invasively programmed into the memory 94 through a telemetrycircuit 100 in telemetric communication with the external device 102,such as a programmer, transtelephonic transceiver, or a diagnosticsystem analyzer. The telemetry circuit 100 is activated by themicrocontroller by a control signal 106. The telemetry circuit 100advantageously allows intracardiac electrograms and status informationrelating to the operation of the device 10 (as contained in themicrocontroller 60 or memory 94) to be sent to the external device 102through an established communication link 104. In the preferredembodiment, the stimulation device 10 further includes a physiologicsensor 108, commonly referred to as a “rate-responsive” sensor becauseit is typically used to adjust pacing stimulation rate according to theexercise state of the patient. However, the physiological sensor 108 mayfurther be used to detect changes in cardiac output, changes in thephysiological condition of the heart, or diurnal changes in activity(e.g., detecting sleep and wake states). Accordingly, themicrocontroller 60 responds by adjusting the various pacing parameters(such as rate, AV Delay, V—V Delay, etc.) at which the atrial andventricular pulse generators, 70 and 72, generate stimulation pulses.(V—V delay is typically used in only connection with independentlyprogrammable RV and LV leads for biventricular pacing.) While shown asbeing included within the stimulation device 10, it is to be understoodthat the physiologic sensor 108 may also be external to the stimulationdevice 10, yet still be implanted within or carried by the patient. Acommon type of rate responsive sensor is an activity sensor, such as anaccelerometer or a piezoelectric crystal, which is mounted within thehousing 40 of the stimulation device 10. Other types of physiologicsensors are also known, for example, sensors that sense the oxygencontent of blood, respiration rate and/or minute ventilation, pH ofblood, ventricular gradient, etc. However, any sensor may be used whichis capable of sensing a physiological parameter that corresponds to theexercise state of the patient.

The stimulation device additionally includes a battery 110, whichprovides operating power to all of the circuits shown in FIG. 2. For thestimulation device 10, which employs shocking therapy, the battery 110must be capable of operating at low current drains for long periods oftime, and then be capable of providing high-current pulses (forcapacitor charging) when the patient requires a shock pulse. The battery110 must also have a predictable discharge characteristic so thatelective replacement time can be detected. Accordingly, the device 10preferably employs lithium/silver vanadium oxide batteries, as is truefor most (if not all) current devices. As further shown in FIG. 2, thedevice 10 is shown as having an impedance measuring circuit 112 which isenabled by the microcontroller 60 via a control signal 114.

In the case where the stimulation device 10 is intended to operate as animplantable cardioverter/defibrillator (ICD) device, it must detect theoccurrence of an arrhythmia, and automatically apply an appropriateantitachycardia pacing therapy or electrical shock therapy to the heartaimed at terminating the detected arrhythmia. To this end, themicrocontroller 60 further controls a shocking circuit 116 by way of acontrol signal 118. The shocking circuit 116 generates shocking pulsesof low (up to 0.5 Joules), moderate (0.5–10 joules), or high energy (11to 40 joules), as controlled by the microcontroller 60. Such shockingpulses are applied to the patient's heart 12 through at least twoshocking electrodes, and as shown in this embodiment, selected from theleft atrial coil electrode 28, the RV coil electrode 36, and/or the SVCcoil electrode 38. As noted above, the housing 40 may act as an activeelectrode in combination with the RV electrode 36, or as part of a splitelectrical vector using the SVC coil electrode 38 or the left atrialcoil electrode 28 (i.e., using the RV electrode as a common electrode).

Cardioversion shocks are of relatively low to moderate energy level (soas to minimize the current drain on the battery) and are usually between5 to 20 joules. Typically, cardioversion shocks are synchronized with anR-wave. Defibrillation shocks are generally of moderate to high energylevel (i.e., corresponding to thresholds in the range of 15 to 40Joules), delivered asynchronously (since R-waves may be toodisorganized), and pertaining exclusively to the treatment offibrillation. Accordingly, the microcontroller 60 is capable ofcontrolling the synchronous or asynchronous delivery of the shockingpulses.

Referring to the remaining figures, flow charts are shown describing anoverview of the operation and novel features of stimulation device 10 asconfigured in accordance with exemplary embodiments of the invention. Inthese flow charts, the various algorithmic steps are summarized inindividual “blocks”. Such blocks describe specific actions or decisionsmade or carried out as the algorithm proceeds. Where a microcontroller(or equivalent) is employed, the flow charts presented herein providethe basis for a “control program” that may be used by such amicrocontroller (or equivalent) to effectuate the desired control of thestimulation device. Those skilled in the art may readily write such acontrol program based on the flow charts and other descriptionspresented herein.

Capture-based Atrial Tachycardia Detection

FIG. 3 provides an overview of a method performed by the capture-basedtachycardia detection unit (item 99 of FIG. 2) for automaticallydetecting atrial tachycardia based on a loss of capture of atrial pacingpulses during preventive overdrive pacing. Initially, at step 200, theoverdrive pacing unit (item 101 of FIG. 2) controls the delivery ofpreventive overdrive pacing to the heart and, at step 202, attempts toverify capture of each overdrive pulse using the capture detection unit(item 105 of FIG. 2), which is configured to verify capture of theatrial overdrive pacing pulses. So long as each pulse is captured,preventive atrial overdrive pacing is performed continuously via steps200 and 202. If a pulse fails to capture (i.e. an LOC occurs), the LOCmay be the result of the onset of an atrial tachycardia. Morespecifically, the sudden increase in atrial rate due to the tachycardiamay have caused the atria to beat before the overdrive pulse could bedelivered, rendering atrial tissue refractory at the time the overdrivepulse was delivered. Hence, the overdrive pulse is not captured and aLOC is detected. On the other hand, the LOC may be merely the result ofthe overdrive pulse having a magnitude set too low to properly evoke adepolarization response. So, at step 204, the stimulation devicedelivers an atrial backup pulse at a higher pulse magnitude and, atstep, 206, attempts to verify capture of the backup pulse (again usingthe capture detection unit). If the backup pulse also fails to evoke aresponse, atrial tachycardia is thereby detected at step 208 andappropriate steps are taken at step 210 to respond to the atrialtachycardia.

Thus, FIG. 3 illustrates a technique wherein the detection of LOCsduring preventive overdrive pacing is exploited to detect an atrialtachycardia. If the preventive overdrive pacing technique being employedis DAO, the atrial tachycardia detected is either an organized atrialtachycardia (AT) or AF and, accordingly, the response provided at step210 is preferably directed to terminating the AT or AF, such asdelivering atrial ATP. If ATP is ineffective, then atrial cardioversionshocks are utilized. In any case, with this technique, atrialtachycardia can be detected promptly so that responsive therapies havethe maximum likelihood of success. At least insofar as AF is concerned,with many conventional AF detection techniques, by the time AF isdetected and ATP is activated it may be too late to reliably terminatethe AF.

Although not shown in FIG. 3, if the preventive overdrive pacing pulsesdelivered at step 200 are delivered at a maximum pulse magnitude, it maynot be necessary to deliver a backup pulse at step 204. Rather, the lossof capture of the primary overdrive pulse may be sufficient to warrantthe conclusion that a tachycardia has commenced. However, it may stillbe desirable to deliver a backup pulse anyway to thereby verify that theloss of capture of the primary pulse was indeed the result of atachycardia.

Automatic DAO/ATP Switching Method

FIG. 4 illustrates an exemplary technique for automatically switchingbetween preventive atrial overdrive pacing and therapeutic ATP upondetection of AF, which particularly highlights techniques fordetermining whether or not a LOC detected during preventive overdrivepacing is the result of AF. FIG. 4 assumes that preventive overdrivepacing has already been activated (preferably employing DAO techniques)and is on-going and that the pulse magnitude for overdrive pacing pulseshas already been set based on a previous automatic stimulation thresholdsearch performed by the capture detection unit (item 105 of FIG. 2). Thepulse magnitude that is set via an automatic stimulation thresholdsearch is typically substantially less than the maximum pulse magnitudethat the stimulation device can deliver. In any case, while preventiveoverdrive pacing is performed in accord with the DAO algorithm, theoverdrive pacing unit (item 101 of FIG. 2) calculates an escape intervalfor use in determining the time window to deliver a next overdrivepacing pulse. During step 302, the device waits during the escapeinterval to determine whether a depolarization occurs. If nodepolarization is detected during the escape interval, the nextoverdrive pacing pulse is delivered at step 304. The automatic captureunit then attempts, at step 305, to verify that the overdrive pulse wasproperly captured and, if so, processing simply returns to step 300 forcalculation of the next escape interval for delivery of the next pulse.However, if the overdrive pacing pulse of step 304 is not captured, abackup pulse is then delivered at step 306 by the overdrive pacing unit.The backup pulse is preferably set to a magnitude considerably greaterthen the magnitude of the overdrive pacing pulse of step 304. Dependingupon the implementation, the back-up pulse may be set to the maximumpulse magnitude the stimulation device can deliver or to some magnitudeintermediate the magnitude of the overdrive pacing pulse of step 304 andthe maximum pulse magnitude. Also, in circumstances wherein the primarypulse is already at the maximum magnitude, a backup pulse maynevertheless be provided, also at the maximum magnitude.

At step 307, the capture detection unit again seeks to verify capture.If the previous overdrive pacing pulse of step 304 failed to evokecapture but the backup pulse of step 306 was properly captured, then theLOC associated with the overdrive pacing pulse was probably not theresult of a AF but was instead probably the result of the overdrivepulse magnitude being set too low. Accordingly, at step 308, anotherautomatic stimulation threshold search is performed (by the automaticcapture unit) for the purposes of determining the new atrial capturethreshold for the patient so that the magnitude of the overdrive pacingpulses can be set properly. However, if even the backup pulse of step306 failed to properly evoke capture, then the heart is most likelysuffering AT or AF and an ATP regimen is immediately initiated at step310 using the ATP unit (item 103 of FIG. 2). In this manner, ATP isimmediately activated upon the detection of an apparent AF so as to havethe best possible chance of terminating AF. The ATP unit may beconfigured to perform any of a variety of ATP techniques, such as burstpacing or scanning ATP or therapeutic overdrive ATP. Once the ATPregimen is complete, processing returns to step 300 for calculation ofthe next escape interval. Note that, if the ATP regimen is notsuccessful, another LOC of a backup pulse will likely be detected againand another ATP regimen promptly delivered. Additional logic may beprovided to ensure that the device does not endlessly activate anddeactivate the ATP unit despite a chronic lack of success.

Returning to step 302, if a depolarization is detected during the latestescape interval, indicating that some type of intrinsic electrical eventoccurred, then the stimulation device determines, at step 311, whetherthe intrinsic event occurred within a period of time at which anintrinsic beat would be expected. More specifically, the overdrive unitcompares the timing of the depolarization against a time windowrepresentative of an expected intrinsic beat. The time window for theexpected intrinsic beat may be calculated in accordance with otherwiseroutine pacing techniques then, preferably, expanded by 30% in length.In any case, if the depolarization is within the expected range, thenthe stimulation device interprets the depolarization to be that of asinus beat and returns to step 300 for detection of the next escapeinterval. If, however, the depolarization did not occur within theexpected time period, the stimulation device concludes that thedepolarization is that of a PAC. Assuming PAC response has been enabled(as verified at step 312), a PAC response protocol is initiated, at step313. Once the PAC response protocol has completed, processing againreturns to step 300.

If PAC response has not been enabled within the device then, at step314, the stimulation device instead waits a period of time, at step 315,to ensure that the heart is no longer refractory (for example 400milliseconds) and then delivers a pacing pulse, at step 316, at themaximum pulse magnitude. (This maximum pulse magnitude is greater thanor equal to the backup pulse magnitude of step 306.) The capture unitattempts to verify capture, at step 318, and if this maximum pacingpulse is properly captured, processing returns to step 300 forcalculation of the next escape interval. If, however, the maximum pacingpulse is not captured even though the heart should have been refractorywhen the pulse was delivered, then AT or AF is presumed and ATP isactivated, at step 310.

With respect to the automatic threshold stimulation search performed atstep 308, rather than trigger the search based on a single pair ofpulses that fail to evoke capture (i.e., a single pacing pulse/backuppulse pair), in alternative implementations the search may instead betriggered based on some number of LOC events occurring within a selectednumber of pacing cycles. For example, the stimulation threshold searchmay instead be triggered only if a programmable number of consecutivepacing pulses do not capture or if “x out of y” pacing pulses do notcapture (such as if four out of the last seven pulses fail to evokecapture). A technique for exploiting an “x out of Y” approach is setforth in U.S. Pat. No. 6,430,441 to Levine, entitled “ImplantableCardiac Stimulation Device Having Autocapture/Autothreshold Capability,”which is incorporated by reference herein.

Thus, FIG. 4 illustrates a technique for automatically activating ATPbased on the failure of pacing pulses to properly evoke capture, whichseeks to quickly verify that an AT or AF is occurring before activatingATP. With this technique, a single ATP regimen is delivered at step 310upon detection of AF, then preventive overdrive pacing resumes.Alternatively, the stimulation device may be configured to continuouslydeliver ATP therapy until AF is terminated or until a predeterminedtime-out period expires. Also, the ATP unit may be configured to cyclethrough different types of ATP regimens in an attempt to terminate AF.For example, if a burst ATP technique is not successful during a firstexecution of step 310, then the next time step 310 is performed the ATPunit may automatically switch to a scanning technique. Also, preventiveoverdrive pacing need not be performed continuously whenever ATP is notperformed. Rather, preventive overdrive pacing may be selectivelyactivated or deactivated and additional logic may be provided forcontrolling the activation or deactivation of preventive overdrivepacing. In one specific example, preventive overdrive pacing may bedeactivated at night or while the patient is asleep to provide for agenerally lower sleeping heart rate. If so, otherwise conventional AFdetection techniques may be employed while preventive overdrive pacingis deactivated to detect AF for the purposes of triggering ATP. As canbe appreciated, FIG. 4 merely provides an overview of technique andnumerous and details and alternatives are not shown.

DAO/ATP Setup Procedure

FIG. 5 provides an overview of a set up technique used to determinewhether to enable the DAO/ATP switching of the technique of FIG. 4within the implantable device. The technique is preferably automaticallyperformed following implantation of the device, though the technique canbe performed later under the control of a physician via commands enteredusing an external programmer and transmitted to the implantable device.Briefly, the technique operates to determine whether a tachycardiaarising subsequent to LOCs during preventive overdrive pacingspontaneously terminate. If such tachycardias spontaneously terminate,then it is not necessary to activate ATP following LOCs. On the otherhand, if most of such tachycardias do not spontaneously terminate, thenATP should be activated as soon as possible following detection of atrue LOC during preventive overdrive pacing in an attempt to revert theheart to a normal rhythm.

Beginning at step 400, the microcontroller of the implantable devicecontrols the overdrive pacing unit to deliver preventive overdrivepacing to the heart of the patient while detecting LOCs. Then, thetachycardia detection unit detects any tachycardias arising subsequentto the LOCs, at step 402. Upon detection of a tachycardia, themicrocontroller does not activate ATP in an attempt to terminate thetachycardias. Rather, the microcontroller waits a predetermined periodof time to determine whether the tachycardia spontaneously terminates.Following completion of a predetermined set up period, which may be, forexample, in the range of one to two months (wherein the range isprogrammable), the microcontroller determines, at step 406, thepercentage of tachycardias that spontaneously terminated out of thetotal number of arrhythmias occurring subsequent to an LOC duringpreventive overdrive pacing. If the percentage falls below apredetermined threshold, which may be, for example, 60%, then automaticDAO to ATP switching is enabled by the microcontroller, at step 408.Otherwise, automatic switching from DAO to ATP is disabled. Thus,automatic switching from DAO to ATP following detection of an LOC duringpreventive overdrive pacing is performed only if most tachycardiasarising under those conditions do not spontaneously terminate. If mostsuch tachycardias do spontaneously terminate, then automatic DAO to ATPswitching is not enabled since most such tachycardias terminatespontaneously and ATP would be unnecessary and, perhaps,counterproductive. Alternatively, rather that repeating steps 400–404for a predetermined set up period, the steps may be performed until apredetermined number of tachycardia episodes have been detected. Thepredetermined number may be, e.g., in the range of 10 to 20 episodes.The number of episodes is programmable by the physician and depends, inpart, on the particular tachycardias to which a patient may besusceptible, as well as rate, stability, onset, and other criteria. Agiven patient may have multiple different tachycardias, which would eachmandate specific yet different therapies. This will likely impact boththe delivered therapies and the likelihood of effectiveness for ATP. Inany case, the physician takes these factors into account in determiningan appropriate number of episodes to program.

In addition to performing the set up technique of FIG. 5 followingimplantation of the device, the technique may be periodically performed,perhaps once ever several months, to verify that no changes haveoccurred within the patient that might affect the percentage ofspontaneously terminating arrhythmias and that therefore might affectthe determination of whether to enable DAO to ATP switching. Suchchanges within the patient may arise as a result of new medicationregimes or perhaps as a result of the progression or regression ofmedical conditions within the patient. Progression of congestive heartfailure, for example, may increase the percentage of tachycardias thatdo not spontaneously terminate. Alternatively, the set up technique canbe performed under the control of a physician whenever appropriate.Thus, for example, if the physician prescribes new medications to thepatient that might affect the spontaneous termination of tachycardias,the physician may wish to control the implantable device to perform theset up process to re-evaluate whether automatic DAO to ATP switchingshould be activated within the device.

Although not explicitly shown in FIG. 5, at step 404, if the tachycardiahas not spontaneously terminated at the end of the time period, ATP isthen activated in an attempt to terminate the tachycardia. Thepredetermined period of time prior to activating ATP may be, forexample, in the range of 15–30 seconds. Note that this relatively longdelay before triggering ATP is employed only during the set-upprocedure—before it is known whether the tachycardias associated withLOC will spontaneously terminate. The relatively long period is providedto give the heart a chance to spontaneously revert to a normal sinusrhythm before ATP is activated. After the set-up procedure has beencompleted (and if it has been determined that the tachycardias do notspontaneously terminate), then DAO to ATP switching is performed muchmore quickly in accordance with the techniques described wherein,typically, ATP is activated following a LOC within only two to threeseconds.

Also, although described in connection with enabling automatic DAO toATP switching, the technique of FIG. 5 need not be limited to the use ofDAO and may instead employ other types of preventive overdrive pacing.Additionally, although not shown in the figure, diagnostic informationpertaining to all episodes of arrhythmia occurring subsequent to an LOCduring preventive overdrive pacing is stored within memory forsubsequent review by the physician via an external programmer. Thus, thephysician can directly review the spontaneous termination percentagedetected by the implantable device and can review IEGM signals recordedfor the individual episodes of tachycardia. In some circumstances, thephysician may wish to override the determination made by the implantabledevice to, for example, enable automatic switching even though thedetected percentage does not exceed the threshold or to disableautomatic switching in other cases. Hence, the determination made viathe setup technique of FIG. 5 is in no way permanent or dispositive.

Capture Verification of Backup Pulses

As explained above, loss of capture of backup pulses during atrialoverdrive pacing may be used to detect an atrial tachycardia so that ATPmay promptly be applied. Loss of capture of backup pulses may be usedfor other purposes as well, such for detecting low amplitude VF based onloss of capture of ventricular backup pulses. Accordingly, FIG. 6 setsforth a general technique for verifying capture of backup pulses,whether atrial or ventricular. Briefly, at step 500, the implantedstimulation device controls the delivery of atrial and/or ventricularpacing pulses to the heart and, at step 502, attempts to verify captureof each pulse using the capture detection unit (item 105 of FIG. 2),which is configured to verify capture of either atrial or ventricularpacing pulses (or both). The pacing pulses may be delivered inaccordance with preventive overdrive pacing techniques or in accordancewith other pacing techniques. So long as each pulse is captured, pacingcontinues via steps 500 and 502. If a pulse fails to capture (i.e. a LOCoccurs), a backup pulse is delivered at step 504 and the capturedetection unit attempts to verify capture of the backup pulse at step506. If the backup pulse also fails to evoke a response, appropriateaction is taken. For example, if the backup pulse was an atrial pulse,then ATP may be delivered at step 508 in accordance with techniquesalready described. If the backup pulse was a ventricular backup pulse,then defibrillation shocks may be delivered to the heart in accordancewith techniques summarized below. However, if the backup pulse wascaptured, action is taken at step 510 to respond to the original loss ofcapture of the primary pulse. In particular, stimulation thresholdsearches may be performed in the atria or ventricles. As alreadyexplained, such searches may be triggered based on a single LOC of aprimary pulse or may be based on some number of consecutive LOCs or somenumber of LOCs out of a predetermined number of beats.

Thus, FIG. 6 summarizes a general technique exploiting captureverification of backup pulses. A wide range of specific implementationsmay be developed in accordance with the general principles of backuppulse capture verification.

Insofar as backup pulses are concerned, see also, U.S. patentapplication Ser. No. 10/340,099 of Gray, filed Jan. 10, 2003, whichprovides for measuring the impedance associated with bipolar backuppulses in circumstances wherein a capture threshold has previously beenfound to have increased. The technique of Gray seeks to detect apossible mechanical problem within the lead but is not directed tocapture verification of the backup pulse itself.

Capture-based Ventricular Tachycardia Detection

As noted in the discussions above, ventricular tachycardia may bedetected based on loss of capture of ventricular backup pulses. FIG. 7summarizes this technique. Briefly, at step 600, the implantedstimulation device controls the delivery of ventricular pacing pulses tothe heart and, at step 602, attempts to verify capture of eachventricular pulse using the capture detection unit (item 105 of FIG. 2).Depending upon the implementation, the pacing pulses may be delivered inaccordance with preventive ventricular overdrive pacing techniques or inaccordance with other pacing techniques. So long as each ventricularpulse is captured, ventricular pacing continues via steps 600 and 602.If a ventricular pulse fails to capture, a ventricular backup pulse isdelivered at step 604 and the capture detection unit attempts to verifycapture of the backup pulse at step 606. If the backup pulse also failsto evoke a response, a possible ventricular tachyarrhythmia is detected,at step 608, such as a low amplitude VF. Steps may then be taken toverify the presence of the VF, such as by increasing a ventricularsensitivity to search for undersensing of ventricular intrinsic events.In any case, if VF is detected, defibrillation shocks are delivered tothe heart at step 610. However, if the ventricular backup pulse wascaptured, action is taken at step 612 to respond to the original loss ofcapture of the primary ventricular pulse. In particular, a stimulationthreshold search may be performed as explained above. As before, suchsearches may be triggered based on a single LOC of a primary pulse ormay be based on some number of consecutive LOCs or some number of LOCsout of a predetermined number of beats.

As can be appreciated a wide variety of techniques can be implementedconsistent with the principles the invention and no attempt is madeherein to describe all possible capture-based tachycardia detectiontechniques, DAO/ATP switching techniques, DAO/ATP set up techniques,backup pulse capture detection techniques or ventricular fibrillationdetection techniques. Although described primarily with reference to anexample wherein the implanted device is a defibrillation/pacer,principles of the invention are applicable to other implanted cardiacstimulation devices as well such as pacemakers without defibrillationcapability. The various functional components of the exemplary systemsmay be implemented using any appropriate technology including, forexample, microprocessors running software programs or applicationspecific integrated circuits (ASICs) executing hard-wired logicoperations. The exemplary embodiments of the invention described hereinare merely illustrative of the invention and should not be construed aslimiting the scope of the invention.

1. In an implantable cardiac stimulation device for implant within apatient, a system comprising: a pacing unit operative to deliver primarypacing pulses and backup pacing pulses to the ventricles of the heart; acapture detection unit operative to detect loss of capture of bothprimary pacing pulses and backup pacing pulses in the ventricles; and acapture-based ventricular tachycardia detection unit operative to detecta ventricular tachycardia based upon loss of capture of a ventricularbackup pulse as detected by the capture detection unit.
 2. The system ofclaim 1 wherein the pacing unit delivers pacing pulses at a pulsemagnitude less than a predetermined maximum pulse magnitude and deliversa backup pulse at the maximum pulse magnitude upon detection of a lossof capture of a primary pacing pulse.
 3. The system of claim 1 furthercomprising: a stimulation threshold search unit operative to determine aventricular capture threshold for primary pacing pulses.
 4. The systemof claim 3 wherein the stimulation threshold search unit is activated ifa programmable number of consecutive pacing pulses do not capture butcorresponding backup pulses do capture.
 5. The system of claim 4 whereinthe stimulation threshold search unit is activated if a firstpredetermined number of pacing pulses do not capture within a secondpredetermined number of delivered pulses.
 6. The system of claim 1further comprising: an shock therapy unit operative to deliver shocktherapy to the ventricles upon the detection of tachycardia by thetachycardia detection unit.
 7. The system of claim 6 wherein the pacingunit is controlled to provide preventive overdrive pacing whenever aventricular tachycardia is not detected and wherein the shock therapyunit is controlled to deliver shock therapy to the ventricles upondetection of a ventricular tachycardia.
 8. The system of claim 1 whereinthe pacing unit delivers pacing pulses at a pulse magnitude less than apredetermined maximum pulse magnitude and delivers a backup pulse at themaximum pulse magnitude upon detection of a loss of capture of a primarypacing pulse, and wherein the capture-based ventricular tachycardiadetection unit is operative to detect a ventricular tachycardia basedupon loss of capture of a single ventricular backup pulse at the maximumpulse magnitude.
 9. The system of claim 1 wherein the pacing unit isoperative to deliver primary pacing pulses and backup pacing pulses tothe heart if no intrinsic depolarization is detected during aventricular escape interval.
 10. The system of claim 1 wherein theventricular tachycardia is a low amplitude ventricular fibrillation. 11.The system of claim 1 wherein the pacing unit is operative to deliverprimary pacing pulses and backup pacing pulses to the ventricles of theheart during ventricular overdrive pacing.
 12. In an implantable cardiacstimulation device having a pacing unit and capture detection unit forimplant within a patient, a method comprising: delivering primary pacingpulses to the ventricles of the heart; verifying capture of the primarypacing pulses; delivering a backup pulse to the ventricles of the heartupon detection of a loss of capture of a primary pacing pulse; verifyingcapture of the ventricular backup pacing pulses; detecting a ventriculartachycardia based upon detection of loss of capture of a backup pulse inthe ventricles as detected by the capture detection unit.
 13. The methodof claim 12 wherein delivering primary pacing pulses is performed todeliver pulses at a pulse magnitude less than a predetermined maximumpulse magnitude and wherein delivering a backup pulse is performed todeliver the backup pulse at the maximum pulse magnitude.
 14. The methodof claim 12 wherein the stimulation device comprises a stimulationthreshold search unit operative to determine a capture threshold forpacing pulses and wherein the method further comprises: performing astimulation threshold search using the stimulation threshold search unitif a primary pacing pulse is not captured but a backup pulse iscaptured.
 15. The method of claim 14 wherein delivering primary pacingpulses to the heart is performed in accordance with preventive overdrivepacing.
 16. The method of claim 12 wherein the stimulation devicecomprises a shock therapy unit operative to deliver shock therapy to theventricles and wherein the method further comprises: delivering shocktherapy to the ventricles if both a primary pacing pulse and a backuppulse are not captured in the ventricles.
 17. The method of claim 12wherein the ventricular tachycardia is a low amplitude ventricularfibrillation.
 18. The method of claim 12 wherein delivering primarypacing pulses to the ventricles of the heart occurs if no intrinsicdepolarization is detected during a ventricular escape interval.
 19. Themethod of claim 12 wherein the ventricular tachycardia is a lowamplitude ventricular fibrillation.
 20. In an implantable cardiacstimulation device for implant within a patient, a system comprising:means for delivering primary pacing pulses to the ventricles of theheart; means for verifying capture of the primary pacing pulses; meansfor delivering a backup pulse to the ventricles of the heart upondetection of a loss of capture of a primary pacing pulse; and means forverifying capture of the ventricular backup pacing pulses; and means fordetecting a ventricular tachycardia based upon loss of capture of aventricular backup pulse.
 21. The system of claim 20 wherein the meansfor delivering primary pacing pulses to the ventricles of the heartoccurs if no intrinsic depolarization is detected during a ventricularescape interval.
 22. The system of claim 20 wherein the means fordetecting a ventricular tachycardia is based upon loss of capture of asingle backup pulse.
 23. The system of claim 20 wherein the primarypacing pulses are delivered at a pulse magnitude less than apredetermined maximum pulse magnitude and the backup pulse is deliveredat the maximum pulse magnitude upon detection of a loss of capture of aprimary pacing pulse, and wherein the means for detecting a ventriculartachycardia is based upon loss of capture of single backup pulse. 24.The system of claim 20 wherein the ventricular tachycardia is a lowamplitude ventricular fibrillation.